A biobattery that breathes metal and could power a porch light

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I’m not sure how it happened, but cars powered by bacteria are pretty much the holy grail of bio-engineering. We could have picked complex tissue regeneration or something, but no; for whatever reason, any implication that we might be able to replace gasoline with bacterial garbage-eaters makes the world’s biologists jump to attention. It’s a gleeful, child-like sort of energy. Even seemingly unrelated discoveries get brought back around to support that old, inherited goal, as when NASA’s allegedly arsenic-based life became fodder for speculation about infection-proof bacterial fuel cells. We really love the idea of turning life into power.

Unfortunately, cars are virtually the worst possible application of bio-power. A single bacterial colony is unlikely to produce enough material (say, ethanol) to power a car every day, and no current-generating colony will be powerful enough to push a 2,000 pound automobile past a semi truck on the freeway. As we’ve seen in prior attempts at direct-power bio-batteries, the old and somewhat quaint idea of bio-powered cars that has been so indelible in the minds of biologists simply must be scaled back. This week researchers announced a breakthrough on the path to doing just that: they’ve come one step closer to harnessing one of the few battery cells we’ve found living in nature.

They’re not really battery cells, per se, since they do seem unusually willing to give up whatever energy they have, and they do so in the form of raw electrons. The species in question, Shewanella oneidensis, was long ago observed reducing heavy metals (forcing them to take electrons) in New York waterways, and its mechanism of achieving that change is what we hope to exploit in the future. The researchers, led by Tom Clarke of the University of East Anglia, think they’ve figured out exactly how Shewanella handles the transfer of these electrons: they breathe metal.

The term “breathe” may be misleading, here. In biological parlance, breathing is the process of using oxygen to allow a continual flow of electrons down each cell’s electron transport chain, the cell’s primary respiratory reaction center. Basically, as the electron is passed along a chain of increasingly greedy proteins, its movements down the chain power the creation of ATP, a cell’s main useable fuel molecule. Once it reaches the end of the chain, however, the electron has to go somewhere or stall the whole process — that’s where oxygen comes in. A free oxygen atom will abscond with the used-up electron and grab two passing hydrogens, making H2O for us to use or excrete at our leisure.

Traditionally, bio-power has come in the form of secreted bio-fuels.

However, what if you decided to use something other than free-floating oxygen? In theory this should be fine, but life has had trouble finding an electron acceptor that’s both strong and abundant enough to do the job. Stuck in the metal-rich waters of central North America, Shewanella has had enough time to develop a reliance on heavy metals for this job, and novel little protrusions on the cell surface allow the tricky bacterium to use these elements without even bringing them inside, first. That’s what makes the organism so potentially useful.

Our best attempts at recycling turn plastic bottles into shirts, a nice gesture that saves us resources without necessarily saving us any energy. To be able to chemically affect our wastes, though — to physically either break them down or change them into something more useful, or less hazardous — is the true goal. Bacteria are the perfect candidates for enacting such changes, since they can move between areas of the target material as needed, and since they will continue to act as long as they’re alive. Though this electron-transfer pathway will always require highly electronegative elements like heavy metals, the ability to force electrons on a substance is one that industry is sure to notice.

Finally, of course, there is the possibility that we could set up a conductive acceptor of our own. If the bacteria could be induced to give their electrons to an acceptor of our choosing, and if that acceptor could integrate all those donations into a single, steady current, then we essentially have a working bio-battery right away. The problem is the volume of power this represents: not much. Any sort of professional generator species would probably need to be engineered to live with an extremely high metabolic rate, churn out energy and, likely, die off quickly as a result.

Still, life seems poised to make inroads to virtually every area of research, from robotics, to computing itself; with enough time, this will surely come to something. While a bio-car is still a bit of a pipe dream, we should keep in mind that cells are able to extract energy from basic substances much, much more efficiently than any human technology. Prior attempts to bring microscopic life into the energy industry have focused on using them for simple fuel production; that’s a different source of power, but hardly a different sort of power. These bacteria use a truly novel pathway, and one that could provide some very interesting utility going forward.

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